Purpose: The purpose of this case study was to determine the feasibility of peer-led telephone counseling for people with type 2 diabetes mellitus related to physical activity (PA) and to establish preliminary efficacy of peer-led telephone counseling for eliciting recommended changes in PA-related cognitions and behaviors for adults with type 2 diabetes mellitus. Methods: A total of 8 adults (5 males and 3 females, aged 59.5 (6.5) years) with type 2 diabetes mellitus completed quantitative self-report measures of aerobic-based PA, resistance training (RT), along with metrics from social cognitive theory (SCT) before and after a 12-week intervention. Qualitative data from weekly peer-led telephone sessions were collected by a peer counselor and subsequently organized into themes and analyzed using a mixed-methods, collective case-study approach. Results: PA behaviors remained relatively constant over the 12-week intervention. Self-efficacy for RT improved (z = -2.03; P = .04). From the peer counselor’s perspective, identifiable inhibitors to PA, which included low self-efficacy and disease condition limitations, were successfully translated into enablers/motivators. Perceived health benefits were frequently reported by the peer counselor as motivators for the participants over the study period. Participants believed peer counseling by telephone influenced their decision to continue to participate in PA. Conclusion: Theory-based, peer-led telephone counseling shows some promise for increasing receptiveness to PA, but had little effect on improving behavior for most participants. Further studies will be necessary to determine the effectiveness and the sustainability of this approach.